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Genetics and Cellular Function
1. 4-1
Genetics and Cellular Function
• DNA and RNA – the nucleic acids
• Genes and their action
• DNA replication and the cell cycle
• Chromosomes and heredity
2. 4-2
Modern Genetics
• Mendelian genetics
– Gregor Mendel
– investigates family patterns of inheritance
• Cytogenetics
– uses techniques of cytology and microscopy to study
chromosomes and their relationships to hereditary traits
• Molecular genetics
– uses biochemistry to study structure and function of DNA
• Genomic medicine
– treatment of genetic diseases through an understanding
of the human genome
7. 4-7
Discovery of the Double Helix
• By 1900: components of DNA were known
– sugar, phosphate and bases
• By 1953: x ray diffraction determined geometry of
DNA molecule
• Nobel Prize awarded in 1962 to 3 men: Watson,
Crick and Wilkins, but not to Rosalind Franklin,
who died of cancer at 37, after discovering the
x-ray data that provided the answers to the double
helix.
8. 4-8
DNA Function
• Genes – genetic instructions for synthesis of
proteins
• Gene – segment of DNA that codes for a
specific protein
• Genome - all the genes of one person
– humans have estimated 25,000 to 35,000 genes
• 2% of total DNA
• other 98% is noncoding DNA
– plays role in chromosome structure
– regulation of gene activity
– no function at all – “junk” DNA
12. 4-12
RNA: Structure and Function
• RNA much smaller cousin of DNA (fewer bases)
– messenger RNA (mRNA) over 10,000 bases
– ribosomal RNA (rRNA)
– transfer RNA (tRNA) 70 - 90 bases
– DNA averages 100 million base pairs
• one nucleotide chain (not a double helix as DNA)
• ribose replaces deoxyribose as the sugar
• uracil replaces thymine as a nitrogenous base
• Essential function
– interprets code in DNA
– uses those instructions for protein synthesis
– leaves nucleus and functions in cytoplasm
13. 4-13
What is a Gene?
• Previous definition - gene - a segment of DNA that carries
the code for a particular protein???
– Body has millions of proteins but only 35,000 genes?
– Small % of genes produce only RNA molecules
– Some segments of DNA belong to 2 different genes
• Current Definition – gene - an information-containing
segment of DNA that codes for the production of a molecule
of RNA that plays a role in synthesizing one or more proteins
• Amino acid sequence of a protein is determined by the
nucleotide sequence in the DNA
14. 4-14
Human Genome
• Genome – all the DNA in one 23-chromosome set
– 3.1 billion nucleotide pairs in human genome
• 46 human chromosomes comes in two sets of 23
chromosomes
– one set of 23 chromosomes came form each parent
– each pair of chromosomes has same genes but different versions
(alleles) exist
• Human Genome Project (1990-2003) identified the
nitrogenous base sequences of 99% of the human genome
– genomics – the comprehensive study of the whole genome and
how its genes and noncoding DNA interact to affect the structure
and function of the whole organism.
15. 4-15
Human Genome
• Findings of Human Genome Project
– Homo sapiens has only about 25,000 to 35,000 genes
• not the 100,000 formerly believed
– genes generate millions of different proteins
• not the old one gene one protein theory
• single gene can code for many different proteins
– genes average about 3,000 bases long
• range up to 2.4 million bases
– all humans are at least 99.99% genetically identical
• 0.01% variations that we can differ from one another in more than 3 million base
pairs
• various combinations of these single-nucleotide polymorphisms account
for all human variation
– some chromosomes are gene-rich and some gene-poor
– we now know the locations of more than 1,400 disease-producing
mutations
• opens the door for a new branch of medical diagnosis and treatment called
Genomic Medicine
• before HGP, we new locations of fewer than 100
16. 4-16
Genetic Code
• body can make millions of different proteins, all from the
same 20 amino acids, and encoded by genes made of just
4 nucleotides (A,T,C,G)
• Genetic code – a system that enables these 4 nucleotides
to code for the amino acid sequence of all proteins
• minimum code to symbolize 20 amino acids is 3 nucleotides
per amino acid
• Base triplet – a sequence of 3 DNA nucleotides that stands
for one amino acid
– codon - the 3 base sequence in mRNA
– 64 possible codons available to represent the 20 amino acids
• 61 code for amino acids
• Stop Codons – UAG, UGA, and UAA – signal the ‘end of the message’,
like a period at the end of a sentence
• Start Codon – AUG codes for methionine , and begins the amino acid
sequence of the protein
17. 4-17
Overview of Protein Synthesis
• all body cells, except sex cells and some immune cells,
contain identical genes.
• different genes are activated in different cells
• any given cell uses 1/3 to 2/3rds of its genes
– rest remain dormant and may be functional in other types of cells
• activated gene
– messenger RNA (mRNA) – a mirror-image copy of the gene is made
• migrates from the nucleus to cytoplasm
• its code is read by the ribosomes
– ribosomes – cytoplasmic granules composed of ribosomal RNA
(rRNA) and enzymes
– transfer RNA (tRNA) – delivers amino acids to the ribosome
– ribosomes assemble amino acids in the order directed by the codons
of mRNA
18. 4-18
Summary of Protein Synthesis
• process of protein synthesis
– DNA mRNA protein
• transcription – step from DNA to mRNA
– occurs in the nucleus where DNA is located
• translation – step from mRNA to protein
– most occurs in cytoplasm
– 15-20% of proteins are synthesized in the nucleus
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21. 4-21
Transcription
• DNA too large to leave nucleus and participate directly in cytoplasmic
protein synthesis
– necessary to make a small mRNA copy that can migrate through a nuclear
pore into the cytoplasm
• Transcription – copying genetic instructions from DNA to RNA
• RNA Polymerase – enzyme that binds to the DNA and assembles the
mRNA
– base sequences TATATA or TATAAA inform the polymerase where to begin
– RNA polymerase opens up the DNA helix about 17 base pairs at a time
– reads base from one strand of DNA
– makes corresponding mRNA
• where it finds C on the DNA, it adds G to the mRNA
• where it finds A on the DNA, it adds U to the mRNA
– RNA polymerase rewinds the DNA helix behind it
– gene can be transcribed by several polymerase molecules at once
– terminator – base sequence at the end of a gene which signals polymerase
to stop
22. 4-22
Transcription
• pre-mRNA – immature RNA produced by transcription
• exons – “sense” portions of the immature RNA
– will be translated to protein
• introns – “nonsense” portions of the immature RNA
– must be removed before translation
• alternative splicing – removing the introns by enzymes and splicing
the exons together into a functional RNA molecule
– one gene can code for more than one protein
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25. 4-25
Translation
• translation – the process that converts the language of
nucleotides into the language of amino acids
• ribosomes - translate sequence of nucleotides into the
sequence of amino acids
– occur mainly in cytosol, on surface of rough ER, and
nuclear envelope
– consists of two granular subunits, large and small
• each made of several rRNA and enzyme molecules
27. 4-27
Translation
• requries the participation of transfer RNA (tRNA)
– small RNA molecule
– coils on itself to form an angular L shape
– one end of the L includes three nucleotides called an anticodon
– other end has binding site specific for one amino acid
– each tRNA picks up specific amino acid from pool of free amino
acids in cytosol
• one ATP molecule is used to bind amino acid to site
• provides energy for peptide bond formation
– some imprecision in codon-anticodon pairing
• takes only 48 different tRNAs to pair with 61 different codons
– ribosome binds and holds tRNA with its specific amino acid
– large ribosomal subunit contains an enzyme that forms peptide bond
that links amino acids together
– first tRNA released from ribosome
– second tRNA temporarily anchors growing peptide chain
– ribosome shifts and third tRNA brings its amino acid to the site
– one ribosome can assemble a protein of 400 amino acids in about 20
seconds
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32. 4-32
Protein Processing and Secretion
• protein synthesis is not finished when the amino
acid sequence (primary structure) has been
assembled.
• to be functional it must coil or fold into precise
secondary and tertiary structure
• Chaperone proteins
– older proteins that pick up new proteins and guides the
new protein in folding into the proper shapes
– helps to prevent improper association between different
proteins
– also called stress proteins or heat-shock proteins
• chaperones produced in response to heat or stress
• help damaged protein fold back into correct functional shapes
34. 4-34
Protein Processing and Secretion
• proteins to be used in the cytosol are likely to be made on
free ribosomes in cytosol
• proteins destined for packaging into lysosomes or secretion
from the cell are assembled on rough ER and sent to golgi
complex for packaging
– entire polyribosome migrated to the rough ER and docks on its
surface
– assembled amino acid chain completed on rough ER
– sent to Golgi for final modification
35. 4-35
Protein Processing and Secretion
• proteins assembled on ER surface
• threads itself through a pore in the ER membrane into cisterna
• modifies protein by posttranslational modification
– removing some amino acid segments
– folding the protein
– stabilizing protein with disulfide bridges
– adding carbohydrates
• when rough ER finished with protein
– pinches off bubblelike transport vesicle coated with clathrin
– clathrin helps select the proteins to be transported in vesicles and helps mold forming
vesicle
– vesicles detach from ER and carry protein to the nearest cisterna of Golgi complex
• vesicles fuse and unloads proteins into Golgi cisterna
• Golgi complex further modifies the protein
• passes from cisterna closest to ER to cisterna farthest from ER
• buds off new coated Golgi vesicles containing finished protein
• some Golgi vesicles become lysosomes
• others become secretory vesicles migrate to plasma membrane, fuse to it, and
release their cell product by exocytosis
37. Gene Regulation
• genes are turned on and off from day to
day
• their products are need or not
• many genes are permanently turned off in
any given cell
4-37
38. Gene Regulation
• example of several ways to turn genes on or off
• mother giving birth to first baby
– hormone prolactin stimulates cells of the mammary glands to begin
synthesizing components of breast milk
– including protein casein – something never secreted before
1. prolactin binds to receptors
- pair of proteins in plasma membrane of mammary cell
1. receptors trigger the activation of a regulatory protein (transcription
activator) in cytoplasm
2. regulatory protein moves into the nucleus and binds to the DNA near
the casein gene
3. the binding enables RNA polymerase to bind to the gene and
transcribe it, producing the mRNA for casein
4-38
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40. 4-40
Synthesizing Compounds Other
Than Proteins
• cells synthesize glycogen, fat, steroids, phospholipids,
pigments, and other compounds
– no genes for these
– synthesis under indirect genetic control
– produced by enzymatic reactions
– enzymes are proteins encoded by genes
• example – testosterone production
– a steroid
– a cell of the testes takes in cholesterol
– enzymatically converts it to testosterone
– only occurs when genes for enzyme are activate
• genes may greatly affect such complex outcomes as
behavior, aggression, and sex drive
41. 4-41
DNA Replication and Cell Cycle
• before cells divide, it must duplicate its DNA so
it can give a complete copy of all its genes to
each daughter cell.
• since DNA controls all cellular function, this
replication process must be very exact
• Law of Complementary Base Pairing – we can
predict the base sequence of one DNA strand if
we know the sequence of the other
– enables a cell to reproduce one strand based on the
information in another
43. 4-43
Steps of DNA Replication
1. Double helix unwinds from histones
2. enzyme DNA helicase opens one short segment of helix at a time
-exposing its nitrogen bases
1. replication fork – the point where the DNA is opened up (like two
separated halves of a zipper)
2. DNA polymerase molecules move along each strand
-read the exposed bases
-matches complementary free nucleotides
1. the two separated strands of DNA are copied by separate
polymerase molecules proceeding in opposite directions
-the polymerase molecule moving toward the replication fork makes a
long, continuous, new strand of DNA
-the polymerase molecule moving away from the replication fork
makes short segments of DNA at a time…DNA ligase joins them
together
44. 4-44
Steps of DNA Replication
6. from the old parental DNA molecule, two new daughter DNA
molecules are made
7. semiconservative replication - each daughter DNA consists of one
new helix synthesized from free nucleotides and one old helix
conserved from the parental DNA
8. new histones are synthesized in cytoplasm
-millions of histones are transported into the nucleus within a few
minutes after DNA replication
-each new DNA helix wraps around them to make a new
nucleosome
9. each DNA polymerase works at a rate of 100 base pairs per second
-would take weeks for one polymerase to replicate one
chromosome
-thousands of polymerase molecules work simultaneously on each
DNA molecule
10. all 46 chromosomes are replicated in 6 - 8 hours
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46. 4-46
Errors and Mutations
• DNA polymerase does make mistakes
– multiple modes for correction of replication errors
– double checks the new base pair and tend to replace
incorrect, biochemically unstable pairs with more stable
correct pairs
– result is only 1 error per 1 billion bases replicated
• mutations - changes in DNA structure due to
replication errors or environmental factors
(radiation, viruses, chemicals)
– some mutations cause no ill effects. others kill the cell,
turn it cancerous or cause genetic defects in future
generations.
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51. 4-51
Mitosis
• cell division in all body cells except the eggs and
sperm
• Functions of mitosis
– development of the individual from one fertilized egg to
some 40 trillion cells
– growth of all tissues and organs after birth
– replacement of cells that die
– repair of damaged tissues
• 4 phases of mitosis
– prophase, metaphase, anaphase, telophase
53. 4-53
Mitosis: Prophase
• chromosomes shorten and thicken coiling into compact rods
– easier to distribute to daughter cells than chromatin
• 46 chromosomes
– two chromatids per chromosome
– one molecule of DNA in each chromatid
• nuclear envelope disintegrates and releases chromosomes into the
cytosol
• centrioles sprout elongated microtubules – spindle fibers
– push centrioles apart as they grow
– pair of centrioles lie at each pole of the cell
• some spindle fibers grow toward chromosomes and attach to the
kinetochore on each side of the centromere
• spindle fibers then tug the chromosomes back and forth until they line
up along the midline of the cell
57. 4-57
Mitosis: Telophase
• chromatids cluster on each side of the cell
• rough ER produces new nuclear envelope
around each cluster
• chromatids begin to uncoil and form chromatin
• mitotic spindle breaks up and vanishes
• each nucleus forms nucleoli
– indicating it has already begun making RNA and
preparing for protein synthesis
58. 4-58
Cytokinesis
• cytokinesis – the division of cytoplasm into two
cells
– telophase is the end of nuclear division but overlaps
cytokinesis
– early traces of cytokinesis visible in anaphase
• achieved by motor protein myosin pulling on
microfilaments of actin in the terminal web of
cytoskeleton
• creates the cleavage furrow around the equator
of cell
• cell eventually pinches in two
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60. 4-60
Timing of Cell Division
Cells divide when:
• they have enough cytoplasm for two daughter cells
• they have replicated their DNA
• adequate supply of nutrients
• are stimulated by growth factor
– chemical signals secreted by blood platelets, kidney cells, and other
sources
• neighboring cells die, opening up space in a tissue to be
occupied by new cells
Cells stop dividing when:
• snugly contact neighboring cells
• when nutrients or growth factors are withdrawn
• contact inhibition – the cessation of cell division in
response to contact with other cells
61. 4-61
Chromosomes and Heredity
• heredity - transmission of genetic characteristics from parent to
offspring
• karyotype - chart of 46 chromosomes laid out in order by size and other
physical features
• 23 pairs – the two members of each pair are called homologous
chromosomes
– 1 chromosome from each pair inherited from each parent
• 22 pairs called autosomes
– look alike and carry the same genes
• one pair of sex chromosomes (X and Y)
– normal female has homologous pair of X chromosomes
– normal male has one X and one much smaller Y chromosome
• diploid – any cell with 23 pairs of chromosomes (somatic cells)
• haploid – contain half as many chromosomes as somatic cells – sperm
and egg cells (germ cells)
• fertilization restores diploid number to the fertilized egg and the
somatic cells arise from it.
63. 4-63
Genes and Alleles
• locus – the location of a particular gene on a chromosome
• alleles - different forms of gene at same locus on two
homologous chromosomes
• dominant allele (represented by capital letter)
– corresponding trait is usually detectable in the individual
– masks the effect of any recessive allele that may be present
– produces protein responsible for visible trait
• recessive allele (represented by lower case letter)
– expressed only when present on both of the homologous
chromosomes
– no dominant alleles at that locus
65. 4-65
Genes and Alleles
• genotype – the alleles that an individual possesses for a
particular trait
– homozygous alleles – two identical alleles for a trait
– heterozygous alleles – different alleles for that gene
• phenotype – an observable trait
– an allele is expressed if it shows in the phenotype of an individual
• genetic counselors – perform genetic testing or refer
clients for tests, advise couples on the probability of
transmitting genetic diseases, and assist people on coping
with genetic disease
• Punnett square shows how 2 heterozygous parents with
cleft chins can have child with uncleft chin
– heterozygous carriers of hereditary diseases such as cystic fibrosis
– both parents healthy
66. 4-66
Multiple Alleles, Codominance,
and Incomplete Dominance
• gene pool - collective genetic makeup of population as a
whole
• multiple alleles - more than two allelic forms for a trait
– 100 alleles are responsible for cystic fibrosis
– 3 alleles for ABO blood types
• IA
, IB
, i alleles for ABO blood types
• codominant - both alleles equally dominant
– IA
IB
= type AB blood
– both are phenotypically expressed
• incomplete dominance
– phenotype intermediate between traits each allele would
have produced alone
70. 4-70
Penetrance and Environmental
Effects• Penetrance – the
percentage of a population with
a given genotype that actually
exhibits the predicted
phenotype
– polydacyly
• Role of environment
– no gene can produce a
phenotypic effect without
nutritional and other
environmental input
– need both the genetic
recipe and the ingredients
– brown eye color requires
phenylalanine from diet to
produce melanin pigment Figure 4.22
Phenotype
(brown eyes)
From diet
(environment)
Phenvlalanine
(amino acid)
Melanin
(pigment)
Genes
(DNA)
Tyrosine
Enzyme 1
mRNA 1
Enzyme 2
mRNA 2
71. 4-71
Dominant and Recessive Alleles
at the Population Level
• common misconception that dominant
alleles must be more common in the gene
pool than recessive alleles
• some recessive alleles, blood type O, are
the most common
• some dominant alleles, polydactyly and
blood type AB, are rare in the population
72. 4-72
Cancer
• benign tumor
– slow growth
– contained in fibrous capsule
– will not metastasize
– usually easy to treat
• malignant tumor – called cancer
– fast growing
– metastasize – give off cells that seed the growth of
multiple tumors elsewhere
• Oncology – medical specialty that deals with both
benign and malignant tumors
• tumor angiogenesis – ingrowth of blood vessels
stimulated by energy-hungry tumors
73. 4-73
Cancer
• Cancers are named for the tissue of origin
– carcinomas – originate in epithelial tissue
– lymphomas – originate in lymph nodes
– melanomas – originate in pigment cells of
epidermis (melanocytes)
– leukemias – in blood forming tissues
– sarcomas – in bone, other connective tissue,
or muscle
• carcinogen – environmental cancer-causing
agents
74. 4-74
Causes of Cancer
• Carcinogens – environmental cancer- causing
agents
– radiation – ultraviolet rays, X-rays
– chemical - cigarette tar, food preservatives, industrial
chemicals
– viruses – human papilloma virus, hepatitis C, and type 2
herpes simplex
• 5 -10% of cancers are hereditary
• carcinogens trigger gene mutations
75. 4-75
Malignant Tumor Genes
• Oncogenes
– causes cell division to accelerate out of control
• excessive production of growth factors that stimulate
mitosis
• the production for excessive growth factor receptors
• Tumor suppressor genes
– inhibit development of cancer
• oppose action of oncogenes
• codes for DNA repairing enzymes
76. 4-76
Lethal Effects of Cancer
• replace functional tissue in vital organs
• steal nutrients from the rest of the body
– cachexia – severe wasting away of depleted
tissues
• weaken one’s immunity
• open the door for opportunistic infections
• often invade blood vessels, lung tissue, or
brain tissue